VITESS Modules to simulate Optical Elements

There are 2 VITESS modules to simulate neutrons in refractive elements:
lense can be used to neutron lenses
prism can be used to simulate prisms and arrays of identiccal prisms.

VITESS Neutron Refraction Lense

The module lense simulates an refractive neutron lense or a stack of such lenses.

The first choice is the 'Number of lenses'. This option is very important due to the very small refractive index of all materials for neutron waves. In order to have appropriate focal distance (10-20 m) you have to use a stack of 5-20 lense usually. If this module simulates only one lense, you have to create a long pipe with a lot of modules 'lense'. This is very uncomfortable and slows doen the simulation. To avoid that, you can simply give the number of lenses you like. The lenses will be situated as given in Figure 2.
Two kinds of lenses are included in the module: a lense with spherical geometry and one with parabolic geometry. The geometry has to be chosen. (See radio button 'Lense surface geometry'). The next step is to describe the geometry of a lense. Four parameters are included in the module 'Cur_radius1', 'Cur_radius2', 'RadiusMain' and 'Thickness' (see figure 1).
Please note, that, if the parameters 'Cur_radius1' and/or 'Cur_radius2' are given as zero, one of the sides of the lense will be a straight vertical line (or plane). The parameters 'Cur_radius1' and/or 'Cur_radius2' can have negative values; in this case the surfaces (parabolic or spherical) will be of convex shape - in contrast to figure 1, where concave shapes are shown.

Figure 1a: spherical lense

Figure 1b: parabolic lense

Please note that the parameters 'Position center Y', 'Position center Z', 'Output frame Y' and 'Output frame Z' usually have to be set to zero. Otherwise the lense center and/or the center point in the output plane will be shifted along axis OY and OZ. If the 'Position center Y', 'Position center Z' are not zero, (only) the lense will be shifted along axis OY and/or OZ. If the 'Output frame Y' and 'Output frame Z' are not zero, a coordinate transformation will take place and all components in the instrument after the lense will be shifted. (as with module frame).

Parameters for module 'lense'

Parameter Unit	Description	Range or Values	Command Option
Number of lenses	number of lenses in the stack	≥1	-I
RadiusMain [cm]	Both lenses: half the diameter of the lense - see Figure 1a and 1b	>0	-c
Thickness [cm]	Both lenses: Thickness of the lense along the central axis	>0	-A
Cur_Radius1 [cm]	Spherical lense: Curvature radius of the upstream surface of the lense (where the neutrons enter) - see Figure 1a Parabolic lense: distance between parabola at y=z=0 and y=z=RadiusMain for the upstream surface of the lense (where the neutrons enter) - see Figure 1b	any	-a
Cur_Radius2 [cm]	Spherical lense: Curvature radius of the downstream surface of the lense (where the neutrons leave) - see Figure 1a Parabolic lense: distance between parabola at y=z=0 and y=z=RadiusMain for the downstream surface of the lense (where the neutrons leave) - see Figure 1b	any	-b
Lense surface geometry	geometry of the lense	'spherical' 'parabolic'	-K
Lense material	material of the lense	'O','CO2','C' 'Be','F','Bi' 'MgO','Pb','MgF' 'SiO2''ZrO2','Mg' ,'Si','Zr','Al'	-i
Delta n	Deviation of the refractive index from 1, i.e. refracive index is n = 1 - Δn used if 'material of the lense'='input' is chosen (cf. text)	>0	-R
wavelength [Å]	wavelength for the refractive index given by the user used if the refractive index is given by the user (cf. text)	>0	-C
Attenuation activation	yes: attenuation inside the lense no: no attenuation inside the lense	'yes' 'no'	-H
Absorption [1/cm]	macroscopic absorption cross-section given by the user used if 'material of the lense'='input' (cf. text)	>=0	-D
Scattering [1/cm]	macroscopic scattering cross-section given by a user	>=0	-Q
Surface roughness [deg]	amplitude of waviness of a rectangular distribution this value is the maximal angle of deviation of the surface normal from the ideal normal.	>=0	-q
position center X, Y, Z [cm]	one lense: center position of the lense several lenses: center position of the stack of lenses. (Due to imperfection of the code, it is currently assumed that the lenses are evenly distributed along the x axis between entrance (x=0) and exit (x position of the output frame))	any	-d -e -k
Inner radius [cm]	Inner radius of a diaphragm at the exit of the lenses (see Figure 3)	>=0	-m
Outer radius [cm]	Outer radius of a diaphragm at the exit of the lenses (see Figure 3)	>0	-M
output frame X, Y, Z [cm]	x-, y- and z-position of the output frame (in the input frame) (see figure 2)	any	-s -t -w
choose formula	formulae for focal distance calculation: thick: exact formula thin: approximation for thin lenses	'thick' 'thin'	-V
wavelength [Å]	wavelength for focal distance calculation	>0	-r
activate flight	yes: propagation to the focal point no: propagation to the position of output frame	'yes' 'no'	-Y
activate visalization	yes: show trajectories within the lense system in a separate visualization tool	'yes' 'no'	-Y
Lense number	Index of the lense used for the visualisation ( (0 - means all lenses)	>=0	-E
Output device	Output device for visualisation: x-windows or postscript file used for unix only	>=0	-E
Number of trajectories	Number of trajectories for the visualisation after the lense	>0	-x
Max X	maximal x value at the ray-tracing picture, 0.0 means auto-calculation	>=0.0	-S
Visual ray-tracing after lense	no: no visualisation XZ: visualisation using a projection to the XZ plane XY: visualisation using a projection to the XY plane	'no' 'XZ' 'XY'	-W
file name	name of the output file, into which the entry and exit positions of the trajectories through all lenses are written i.e. color and x,y,z position of entry into lense 1, of exit from lense 1, of entry into lense 2, of exit from lense 2, ...	-	-p
Lense number	Index of the lense used to write data to the output file (0 - means all lenses)	>=0	-v
activate output	yes: activate output of the coordinates for the lense no: no output written	'yes' 'no'	-z

VITESS Neutron Refraction Prism Array

The module prism simulates a refractive neutron prism or a stack of such prisms.

The prism dimensions should be given as input: the base width and its height and the prism height. The orientation of the prism is such that the base is on the x-z plane. The critical angle of the prism is therefore fixed by its height and base dimensions. In order to define a stack of prisms, the user should give as input the number of columns and rows forming the stack. This option is very important due to the very small refractive index of all materials for neutron waves. The prism can optionally attenuate neutron trajectories by setting the property 'Layer can absorb' to 'yes'. The simulation will then take into account the attenuation due to absoription cross section. A depiction of reflection and refraction at a material surface is provided in the figure taken from EPJ Web of Conferences 236, 04001 (2020). As described in figure, the refraction index of neutrons through materials follows the law:

n = 1 - (λ ² / 2 π) ρ

Last modified: Tue May 8 17:08:06 MET DST 2001